1. Field of the Invention
The present invention relates to an exhaust gas purification system for an internal combustion engine.
2. Description of the Related Art
An internal combustion engine is known in which a NOx selective reduction catalyst is disposed in an exhaust passage of the engine and supplied with aqueous urea solution. The NOx selective reduction catalyst adsorbs ammonia that is produced from the urea solution, and selectively reduces NOx in exhaust gases with adsorbed ammonia (see Japanese Patent Application Publication No. 2003-293737 (JP-A-2003-293737), for example). When NOx is reduced with ammonia, which is adsorbed as described above, an amount of ammonia as large as possible is preferably adsorbed on the NOx selective reduction catalyst in order to attain favorable NOx reduction efficiency. In this internal combustion engine, a target ammonia adsorption amount that is slightly less than a saturated ammonia adsorption amount is set in advance, and a supply amount of the urea solution is controlled such that the amount of ammonia that is adsorbed on the NOx selective reduction catalyst equals the target ammonia adsorption amount.
From studies on ammonia adsorption onto the NOx selective reduction catalyst such as described above, it is concluded that there exist at least two types of an ammonia adsorption state. Specifically, in a first adsorption state, when a temperature of the NOx selective reduction catalyst is in a low-temperature region such as below 200° C., ammonia is adsorbed onto the NOx selective reduction catalyst, and upon a temperature increase in the NOx selective reduction catalyst, an ammonia desorption amount reaches maximum in the low-temperature region. In a second adsorption state, when the temperature of the NOx selective reduction catalyst is in a high-temperature region such as above 200° C., ammonia is adsorbed onto or has already been adsorbed on the NOx selective reduction catalyst, and upon the temperature increase in the NOx selective reduction catalyst, the ammonia desorption amount reaches maximum in the high-temperature region. The studies revealed that ammonia that is desorbed from the first adsorption state is more likely to pass through the NOx selective reduction catalyst without purifying NOx than ammonia in that is desorbed from the second adsorption state.
In other words, almost all ammonia that passes through the NOx selective reduction catalyst and is discharged to the atmosphere is in the first adsorption state. Thus, in order to control an amount of ammonia that is discharged to the atmosphere, it is necessary to control an adsorption amount of ammonia in the first adsorption state.
However, as in the above known internal combustion engine, if the total ammonia adsorption amount is controlled, the adsorption amount of ammonia in the first adsorption state may account for a substantial portion of the total ammonia adsorption amount. In such a case, if a vehicle is accelerated, and thereby the temperature of the NOx selective reduction catalyst exceeds the temperature in the low-temperature region, a large amount of ammonia in the first adsorption state is desorbed. Consequently, the large amount of ammonia is discharged to the atmosphere.
In order to solve the problem as described above, it is necessary to control the adsorption amount of ammonia in the first adsorption state, and consequently, it is necessary to estimate the adsorption amount of ammonia in the first adsorption state.